This invention relates to a process and an apparatus for correcting the measuring signals of a fiber optic gyro.
From German Patent document DE-OS 39 26 313, it is known that fiber optic gyros can be operated close to the quadrature point by means of 4.times.4-couplers without the requirement of an additional phase modulator. In the range of this quadrature point, the output signals of the detectors, usually photo diodes, have a sinusoidal dependence on the Sagnac-phases which determine the rotational speed of the gyro so that such gyros have a maximal sensitivity particularly in the case of low rotation rates.
The 4.times.4-coupler of such a fiber optic gyro comprises four adjoining mono-mode optical fibers which are fused to one another and have four gates or arms on both sides of its coupling range. On one side of its coupling range, a light source, preferably an ELED, is connected with one arm, and one detector is respectively connected with at least two other arms, and as a rule, all three remaining arms. On the other side of the coupling range, the two ends of a fiber coil, which is wound from a mono-mode optical fiber, are connected to two arms. The other two arms remain free. These free arms of the coupler on the fiber-coil-side may be connected with monitoring circuits by means of which, for example, the intensity of the light source is tested and is taken into account in the measuring results. A special development of these monitoring circuits is not described in the above-mentioned German Patent document DE-OS 39 26 313.
The operation of the fiber gyro is as follows. The light of the light source is beamed into one of the four input arms of the 4.times.4-coupler and is uniformly distributed by the coupler to the two output arms to which the ends of the fiber coil are connected. The resulting split light waves pass through the mono-mode fiber coil in opposite directions and are guided back into the two arms of the coupler. In the interior coupling range of the coupler, these light portions interfere with one another and the result is a phase/amplitude transformation. In this manner, the Sagnac phase shift of the light waves, which circulate in opposite directions in the fiber coil, caused by a rotation of the gyro is changed into alterations of intensity on the coupler arms on the light source side. These alterations of intensity can be detected by photo diodes. Without being influenced by rotations of the gyro, a portion of the light beamed into the one arm leaves the coupler on the two coil-side free output arms.
The output signals of the photo diodes as a function of the Sagnac phase .phi..sub.S and the thus determined rotational speed as well as the intensity .phi..sub.O of the light source are obtained by: EQU P.sub.i :=(A.sub.i +B.sub.i.multidot. cos(.phi..sub.S +C.sub.i)).multidot.I.sub.O (i=2,3,4) (1)
wherein the gyro coefficients A.sub.i, B.sub.i and C.sub.i are functions of the coupling constants K.sub.ij between the individual light channels in the coupling range, with i and j being from 1 to 4. These coefficients may be determined in a gauging procedure in that the fiber gyro is set into rotation on a gauged revolving table at several known rotational speeds. The output signals P.sub.i as a function of the known rotational speed are measured and recorded. Finally, the unknown gyro coefficients are calculated, preferably by means of a so-called least square fit. The thus obtained coefficients are then stored in an analysis circuit, for example, within a computer. By means of the now known coefficients, the respective rotational speed can be determined by the analysis of the above-mentioned equation (1).
The problem which occurs in this case is that the center wave length of the light source, for example, of an edge-emitting luminescent diode of a superluminescent diode or of a laser diode, varies because of temperature influences. However, a variation of the wavelength also results in a variation of the "coupling constants" in the 4.times.4-coupler with an increase of the wavelength causing a stronger coupling. The above gyro coefficients are therefore also a function of the wavelength. If these coefficients at a certain desired wavelength were determined by means of the gauging procedure, then the analysis of the above equation (1) by means of these coefficients results in an incorrect rotational speed if, during the measurement, the light source has a wavelength which deviates from the desired wavelength.
For a correct determination of the rotational speed, the center wavelength of the light source would therefore have to be stabilized to the desired wavelength adjusted during the gauging procedure. The wavelength of a semiconductor light source, such as a laser diode or a luminescent diode, constructed on a chip may be adjusted by the variation of the chip temperature, for example, by means of a Peltier element, and by the variation of the operating or injection current. It would therefore be obvious to stabilize the chip temperature, for example, by means of a combination of a Peltier element for heating and cooling and a thermal resistor for determining the chip temperature, and, in addition, to keep the injection current constant. However, the application range of fiber gyros frequently includes such a broad temperature range, for example, between approximately -60.degree. C. and +120.degree. C. that, when the dimensions are reasonable, the efficiency of a Peltier element is no longer sufficient for stabilizing the chip within the whole exterior temperature range to a desired temperature.
There is therefore needed a process and apparatus by means of which an easy compensation of the errors of the output signals may be carried out, in which case, this compensation may take place over a wide temperature range.
According to the present invention, this need is met by a process and apparatus for correcting the measuring signals of a fiber optic gyro which measures the rotational speed and which comprises a 4.times.4-coupler consisting of four adjoining optical fibers which are fused to one another and have four arms at both sides of its coupling range. A light source, preferably a semiconductor light source, such as a laser or the like, radiates with a specified desired wavelength. The light source is connected on one side of the coupling range with one arm, and one measuring detector respectively is connected with at least two other arms. On the other side of the coupling range, two arms are connected with the two ends of a fiber coil wound from a mono-mode optical fiber. The measuring signals which were falsified by the temperature-caused drift of the wavelength of the light source, are corrected. The output signals on the two free arms of the coupler on the fiber coil side are measured at the desired wavelength and at the respective actual wavelength and a desired or actual ratio is determined from the respective output signals. The temperature and/or the operating current of the light source are controlled such that the actual ratio is equal to the desired ratio.
This invention is based on the consideration that, because of the above-mentioned constructions, a variation of the chip temperature must take place as a function of the outside temperature, which, in turn, results in a variation of the center wavelength. This variation may either be compensated by a corresponding variation of the injection current or may also be taken into account in the signal analysis. Under these conditions, an exact measurement of the actually existing wavelength is therefore a prerequisite for keeping the wavelength constant. By means of the present invention, a simple measuring of the center wavelength of the radiation coupled from the light source into the coupler is therefore possible for the purpose of the stabilization.
The solution found by means of the present invention is simple in that the output signals on the free arms of the coupler on the fiber coil side are utilized. The output signals P.sub.10 and P.sub.40 of the monitoring diodes connected with the free arms of the coupler are obtained by: EQU P.sub.10 (.lambda.)=S.sub.10 (K.sub.ij (.lambda.)).multidot.I.sub.O (2) EQU P.sub.40 (.lambda.)=S.sub.40 (K.sub.ij (.lambda.)).multidot.I.sub.O (3)
These output signals are, in each case, a product of a transmission function S.sub.10 or S.sub.40 and of the intensity I.sub.O of the light source. The transmission functions are, in turn, a function of the coupling constants K.sub.ij which themselves are a function of the wavelength .lambda.. Thus, the output signals P.sub.10 and P.sub.40 also depend on the wavelength.
As mentioned above, in the case of optical directional couplers of the concerned type, as the wavelength increases, the coupling between adjacent channels (the optical fibers fused to one another) in the coupling region of the coupler will also influenced. On the other hand, the luminous power which is coupled from the channel, which is directly connected to the light source, into a neighbored or adjacent channel, in this case the one free arm of the 4.times.4-coupler on the fiber coil side, will increase with increasing wavelength; on the other hand, the power, which is transmitted from the light source to the other free arm on the fiber coil side directly via the common optical fiber or channel, decreases; see for example D. Mortimore, Electron. Lett. 25, 1989, page 682.
This important recognition is based on the fact that the measuring signals will be correct when the quotient of the output signals on the two free arms of the coupler on the fiber coil side remains constant. The quotient for the actual wavelength is therefore compared with the quotient for the desired wavelength which was determined by means of the above-mentioned gauging procedure. By controlling the injection current of the light source and/or its temperature, for example, by controlling the current of a Peltier element, the center wavelength of the radiation emitted by the light source is controlled such that the measured actual quotient is equal to the stored desired or reference quotient.
By means of these measures, a correction of the measuring signals is possible also in the case of a variation of the center wavelength within a wide temperature range.
In order to also take into account fluctuations of the intensity of the light source, during the signal analysis, the output signals of the measuring detectors on the side of the light source may be standardized by relating them to the output signal of one of the monitoring detectors on the fiber coil side. The quotient of the these two output signals is then no longer a function of the intensity of the light source. Therefor, the Sagnac phase which is to be determined from this quotient will also no longer depend on the intensity of the light source.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.